Sort versions by fields, rather than the whole string.

Without this commit, the sorted list would be:

    0.1.0
    0.10.0
    0.2.0
    ...
    0.9.9

With this commit, the order is fixed:

    0.1.0
    0.2.0
    ...
    0.9.9
    0.10.0

It's enough to sort 3 fields now as Node version number has 3 fields.
Merge pull request #5 from dochang/sort-versions

Sort versions by fields, rather than the whole string.